This invention relates generally to catalysts for destruction of volatile organic compounds (VOCs).
Contamination of the environment by volatile organic compounds (VOCs) is of great concern. VOCs originate in many ways, including spray painting and engine maintenance (degreasing and fuel line repair), indoor air decontamination, dry cleaning, food processing (grills and deep fryers), fume hoods, residential use and solvent-intensive industrial processes. VOCs have direct and secondary (e.g. photochemical smog) effects on health and the environment.
Direct methods for removing VOCs from contaminated air require heating the air stream to relatively high temperatures to incinerate the contaminants. The cost required to maintain such elevated temperatures (around 815 to 925xc2x0 C.) and to cool the surroundings can be unacceptably high.
Various catalysts have been studied for their use in lower-temperature removal of VOCs (see, for example, Spivey, J. J. (1987) Ind. Eng. Chem. Res. 26:2165-2180). The catalytic properties of ceria and CeO2 containing materials have been reviewed (Trovarelli, A. (1996) Catal. Rev. Sci. Eng. 38(4):439-520; Bernal, S. et al. (1999) Catal. Today 50:175; Trovarelli, A. et al. (1999) Catal. Today 50:353-356). Pure CeO2 is reported to be active for the oxidation of CO over the temperature range of 200xc2x0 to 500xc2x0 C. (Liu, W. and Flytzani-Stephanopoulos, M. (1995) J. Catal. 153:304-316). However, temperatures of at least 300xc2x0 C. were reported to be necessary for methane oxidation using compositions such as Ce0.2Zr0.8Ox, Cu0.08[Ce(La)]0.92Ox, Cu0.5[Ce(La)]0.5Ox and Cu0.5Zr0.5Ox (where Ce(La) indicates cerium containing 1.5 wt % lanthanum) (Liu, W. and Flytzani-Stephanopoulos, M. (1995) J. Catal. 153:304-316).
In a mixture of CO and NO, CeO2 can simultaneously oxidize CO and reduce NO to produce CO2 and N2. This quality has led to the application of CeO2 in three-way catalysts for automotive exhaust systems. In this capacity, CeO2 is used in combination with other metals to remove unburned hydrocarbons, CO, and NO (Trovarelli, A. (1996) Catal. Rev. Sci. Eng. 38(4): 439-520).
Cerium oxide doped with 10% Zr, 50% Zr, 4.5% La or 10% La, and cerium oxide (doped with Zr or La as above)-supported silver (2-5% Ag) or copper (5-15%) catalysts have been studied for the combustion of methane (Kundakovic, Lj. and Flytzani-Stephanopoulos, M. (1998) J. Catal. 179:203-221). Temperatures of at least 400xc2x0 C. were required for any appreciable oxidation of methane using these materials.
CeO2xe2x80x94ZrO2 mixed oxides in proportions of Ce0.92Zr0.08O2 to Ce0.5Zr0.5O2, and Ce0.76Zr0.19(Mn or Cu)0.05O2xe2x88x92x, have been reported for use in oxidation of C1-C4 hydrocarbons (Terribile, D. et al. (1999) Catal. Today 47:133-140). Temperatures of about 650 to about 800K were reported to be required for 50% conversion of methane, ethane, propane, butane and isobutane.
U.S. Pat. No. 5,283,041 describes a ternary compound of vanadium oxide (0.01 to 20 weight percent), zirconium oxide (40 to 88 weight percent) and either manganese oxide, cerium oxide and cobalt oxide (3 to 48 weight percent). Platinum, palladium and rhodium may also be present from 0.01 to 5 weight percent. This compound is used to treat gas streams containing halogenated organic compounds using temperatures of 175xc2x0 C. to 550xc2x0 C.
U.S. Pat. No. 5,491,120 describes an inert carrer with a BET surface area of at least 10 m2/g used to support a coating of bulk ceria with a BET surface area of at least 10 m2/g and one or more bulk metal oxides which are either titania, zirconia, ceria-zirconia, silica, alumina-silica or xcex5-alumina metal oxide for oxidizing components of a gas stream, in particular a diesel engine exhaust.
U.S. Pat. No. 5,851,948 describes an inert porous support such as alumina or silica having a surface area of 10-400 m2/g used to support a catalyst composition of 0.5 to 15 wt % of chromium, cobalt, copper, cerium or iron oxide covered with a thin (at most 0.1 mm thick) layer of Pd, Pt, Ru or Rh (concentration of 0.01 to 2 wt %).
U.S. Pat. No. 5,627,124 describes a composition of ceria and alumina (each having a surface area of at least 10 m2/g and being 5 to 95 percent of the composition) coated on a refractory carrier for pollution abatement of exhausts containing unburned fuel or oil.
U.S. Pat. No. 5,882,616 describes perovskites of the formula XYO3, where X is lanthanum, cerium or yttrium, and Y is a transition metal such as copper, chromium, manganese, iron, cobalt and nickel supported on a support such as alumina, silica, magnesium aluminate, titanium oxide, zirconium oxide and mixtures thereof.
Even with somewhat advanced materials, the technologies currently used for reduction of VOCs from contaminated air still require heating the air stream to relatively high temperatures to reduce the concentration of VOCs.
There remains a need for materials and processes that destroy VOCs at low concentrations at ambient- or near-ambient temperatures under oxygen-rich conditions. These materials should have high oxygen storage capacity, optimal surface oxygen mobility, stability against feedstream poisons, active sites with a propensity for xcfx80-bond systems as well as VOCs in general, and should not require high temperatures to reduce the concentration of VOCs in air.
Catalyst compositions for destruction of VOCs comprising: at least one of cerium and zirconium with at least one member of the group consisting of: Gd, La, Sc, Cr, Ti, V, Mn, Fe, Co, Ni, Au, Ag, Cu Pt, Pd, Rh, Ru, Re, Os and Ir are provided.
Catalyst compositions of the invention may comprise mixed oxides, single-phase materials, or multi-phase materials. Catalyst compositions further comprising a supporting material are also provided.
One class of compounds of the invention include catalyst compositions for destruction of VOCs in a gas comprising one or more first metals selected from the group consisting of: Ce and Zr; and at least one of ((a) (b) or (c)) where (a) is one or more second metals selected from the group consisting of: Gd, La, Sr and Sc; (b) is one or more third metals selected from the group consisting of: Ti, V, Mn, Fe, Co, Cr, Ni, Au, Ag and Cu; and (c) is one or more fourth metals selected from the group consisting of Pt, Pd, Rh, Ru, Re, Os and Ir.
In this class of compounds, preferably the second metals are one or more of Gd, Sr and La. Preferably the third metals are one or more of Cu, Au, Fe, Co and Mn. The second and third metals together preferably comprise less than about 80% of said catalyst composition, more preferably together comprise less than about 50% of the catalyst composition, and most preferably together comprise between about 5 and 50% of the catalyst composition. Preferably the fourth metals are Pt, Pd and mixtures thereof, with Pt being more preferred than Pd. The fourth metals preferably comprise less than about 25% of said catalyst composition, and more preferably comprise less than about 5% of said catalyst composition, and most preferably comprise less than about 2% of said catalyst composition.
One class of compounds of the invention has one member from the group of second and third metals combined. Another class of compounds of the invention has one member from the group of second metals and one member from the group of third metals. Another class of compounds of the invention has more than one member from the group of second metals. Another class of compounds of the invention has more than one member from the group of third metals. Another class of compounds of the invention has more than one member from the group of second and third metals combined. Each of the above classes may optionally contain one or more members of the group of fourth metals.
Another class of compositions provided are the catalyst compositions with the formula:
nN/Ce1xe2x88x92xZrcAaAxe2x80x2axe2x80x2Axe2x80x3axe2x80x3BbBxe2x80x2bxe2x80x2Bxe2x80x3bxe2x80x3O2xe2x88x92xcex4
wherein n is a percentage from 0 to 25; N is one or more metals selected from the group consisting of Pt, Pd, Rh, Ru, Re, Os and Jr; x=a+axe2x80x2+axe2x80x3+b+bxe2x80x2+bxe2x80x3+c; a, axe2x80x2, axe2x80x3, b, bxe2x80x2, bxe2x80x3 and c are each, independently of one another, 0 to 0.9; xcex4 is a number which renders the composition charge neutral; A, Axe2x80x2 and Axe2x80x3 are independently selected from the group consisting of Gd, La, Sr and Sc; B, Bxe2x80x2 and Bxe2x80x3 are independently selected from the group consisting of Ti, V, Mn, Fe, Co, Cr, Ni, Au, Ag and Cu; provided that when n is zero, at least one of a, axe2x80x2, axe2x80x3, b, bxe2x80x2, bxe2x80x3 or c is nonzero.
In this class of compounds, preferably, N is selected from the group consisting of Pt and Pd. Preferably, n is a percentage from 0 to 10, more preferably, n is a percentage from 0 to 5, and most preferably, n is a percentage from 0 to 2. In this class of compounds, preferably A, Axe2x80x3 and Axe2x80x3 are selected from the group consisting of Gd, La and Sr. In this class of compounds, preferably B, Bxe2x80x3 and Bxe2x80x3 are selected from the group consisting of Mn, Cu, Fe, Co and Cr. Preferably, c is between 0 to 0.2. Preferably, a+axe2x80x2+axe2x80x3 is between 0 and 0.1. Preferably b+bxe2x80x2+bxe2x80x3 is between 0.05 and 0.5.
Specific examples of this class of compounds include catalyst compositions of formula:
Ce1xe2x88x92fCufO2xe2x88x92xcex4
wherein f is about 0.001 to about 0.5 and xcex4 is a number which renders the composition charge neutral. Specific examples of compounds of this formula include Ce0.9Cu0.1O2xe2x88x92xcex4 and Ce0.8Cu0.2 O2xe2x88x92xcex4. Other specific examples of this class of compounds include catalyst compositions with formula Ce1xe2x88x92gMngO2xe2x88x92xcex4, wherein g is about 0.001 to about 0.8. Specific examples of compounds with this formula include catalyst composition of formula: Ce0.8Mn0.2O2xe2x88x92xcex4 and Ce0.5Mn0.5O2xe2x88x92xcex4. Other specific examples of this class of compounds include catalyst compositions with formula: Ce0.5Fe0.1Cu0.4O2xe2x88x92xcex4, Ce0.475Zr0.05Mn0.475O2xe2x88x92xcex4, Ce0.45Zr0.05Mn0.45Cu0.05O2xe2x88x92xcex4 and Ce0.8Zr0.05Cu0.15O2xe2x88x92xcex4.
Another class of compounds included in the invention are those having formula:
nN/Ce1xe2x88x92xZrcAaAxe2x80x2axe2x80x2BbBxe2x80x2bxe2x80x2O2xe2x88x92xcex4
wherein n is a percentage from 0.01 to 15 N is one or more metals selected from the group consisting of Pt, Pd, Rh, Ru, Re, Os and Ir; x=a+axe2x80x2+b+bxe2x80x2+c; a, axe2x80x2, b, bxe2x80x2 and c are each, independently of one another, 0 to 0.5; xcex4 is a number which renders the composition charge neutral; A and Axe2x80x2 are independently selected from the group consisting of Gd, La, Sr and Sc; B and Bxe2x80x2 are independently selected from the group consisting of Ti, V, Mn, Fe, Co, Cr, Ni, Au, Ag and Cu; provided that when n is zero, at least one of a, axe2x80x2, b, bxe2x80x2 or c is nonzero.
In this class of compounds, preferably, N is Pt or Pd, or mixtures thereof. Preferably, n is a percentage from 0.01 to 10, more preferably, n is a percentage from 0.01 to 5, and most preferably, n is a percentage from 0.01 to 2. In this class of compounds, preferably A and Axe2x80x2 are one or more of Gd, La and Sr, and preferably B and Bxe2x80x2 are one or more of Mn, Cu, Co, Cr and Fe.
Specific examples of catalyst compositions of this class include 5%Pt/Ce0.8Cu0.2O2xe2x88x92xcex4, 1%Pt/Ce0.8Cu0.2O2xe2x88x92xcex4, 5%Pt/Ce0.8Sr0.2O2xe2x88x92xcex4, 1%Pt/Ce0.8Sr0.2O2xe2x88x92xcex4, 5Pt/Ce0.8Au0.2O2xe2x88x92xcex4, 1%Pt/Ce0.8Au0.2O2xe2x88x92xcex4, 5%Pt/CeO2xe2x88x92xcex4 and1%Pt/CeO2xe2x88x92xcex4.
Another class of compounds of this invention include those having formula:
nN/m Ce1xe2x88x92xAaAxe2x80x2axe2x80x2BbBxe2x80x2bxe2x80x2O2xe2x88x92xcex4/Zr1xe2x88x92zAxe2x80x3axe2x80x3Axe2x80x2xe2x80x3axe2x80x2xe2x80x3Bxe2x80x3bxe2x80x3Bxe2x80x2xe2x80x3bxe2x80x2xe2x80x3O2xe2x88x92xcex4
wherein n is a percentage from 0 to 15; m is a percentage greater than 0; N is one or more metals selected from the group consisting of Pt, Pd, Rh, Ru, Re, Os and Ir; x=a+axe2x80x2+b+bxe2x80x2; z=axe2x80x3+axe2x80x2xe2x80x3+bxe2x80x3+bxe2x80x2xe2x80x3; a, axe2x80x2, axe2x80x3, axe2x80x2xe2x80x3, b, bxe2x80x2, bxe2x80x3 and bxe2x80x2xe2x80x3 are each, independently of one another, 0 to 0.5; xcex4 is a number which renders the composition charge neutral; A, Axe2x80x2, Axe2x80x3 and Axe2x80x2xe2x80x3 are independently selected from the group consisting of Gd, La, Sr and Sc; B, Bxe2x80x2, Bxe2x80x3 and Bxe2x80x2xe2x80x3 are independently selected from the group consisting of Ti, V, Mn, Fe, Co, Cr, Ni, Au, Ag and Cu; provided that when n is zero, at least one of a, axe2x80x2, axe2x80x3, axe2x80x2xe2x80x3, b, bxe2x80x2, bxe2x80x3 or bxe2x80x2xe2x80x3 is nonzero.
In this class of compounds, preferably, N is selected from the group consisting of: Pt and Pd and mixtures thereof. Preferably, n is a percentage from 0.01 to 10, more preferably, n is a percentage from 0.01 to 5, and most preferably, n is a percentage from 0.01 to 2. Preferably m is a percentage from 0.5 to 25. In this class of compounds, preferably A, Axe2x80x2, Axe2x80x3 and Axe2x80x2xe2x80x3 are independently selected from the group consisting of Gd, La and Sr, and B, Bxe2x80x2, Bxe2x80x3 and Bxe2x80x2xe2x80x3 are independently selected from the group consisting of Mn, Cu, Co, Cr and Fe.
A specific example of compounds of this class include those of formula: 1% Pt/12%CeO2/ZRO2.
Another class of compounds of this invention include those having formula:
nN/m(CeO2)/p(AaAxe2x80x2Aaxe2x80x2Axe2x80x3axe2x80x3BbBxe2x80x2bxe2x80x2Bxe2x80x3bxe2x80x3O2xe2x88x92xcex4)/q(Axe2x80x2xe2x80x3axe2x80x2xe2x80x3Axe2x80x3xe2x80x3axe2x80x3xe2x80x3Axe2x80x2xe2x80x3xe2x80x3axe2x80x2xe2x80x3xe2x80x3Bxe2x80x2xe2x80x3bxe2x80x2xe2x80x3Bxe2x80x3xe2x80x3bxe2x80x3xe2x80x3Bxe2x80x2xe2x80x3xe2x80x3bxe2x80x2xe2x80x3xe2x80x3O2xe2x88x92xcex4)
wherein n, p and q are percentages from 0 to 50; m is a percentage greater than 0; N is one or more metals selected from the group consisting of Pt, Pd, Rh, Ru, Re, Os and Ir; a, axe2x80x2, axe2x80x3, axe2x80x2xe2x80x3, axe2x80x3xe2x80x3, axe2x80x2xe2x80x3xe2x80x3, b, bxe2x80x2, bxe2x80x3, bxe2x80x2xe2x80x3, bxe2x80x3xe2x80x3 and bxe2x80x2xe2x80x3xe2x80x3 are each, independently of one another, 0 or 1; xcex4 is a number which renders the composition charge neutral; A, Axe2x80x2, Axe2x80x3, Axe2x80x2xe2x80x3, Axe2x80x3xe2x80x3 and Axe2x80x2xe2x80x3xe2x80x3 are independently selected from the group consisting of Gd, La, Sr and Sc; B, Bxe2x80x2, Bxe2x80x3, Bxe2x80x2xe2x80x3, Bxe2x80x3xe2x80x3 and Bxe2x80x2xe2x80x3xe2x80x3 are independently selected from the group consisting of Ti, V, Mn, Fe, Co, Cr, Ni, Au, Ag and Cu; provided that when n is zero, at least one of p and q is nonzero and at least one of a, axe2x80x2, axe2x80x3, axe2x80x2xe2x80x3, axe2x80x3xe2x80x3, axe2x80x2xe2x80x3xe2x80x3, b, bxe2x80x2, bxe2x80x3, bxe2x80x2xe2x80x3, bxe2x80x3xe2x80x3 and bxe2x80x2xe2x80x3xe2x80x3 is nonzero.
In this class of compounds, preferably N is selected from the group consisting of Pt and Pd and mixtures thereof. In this class of compounds, A, Axe2x80x2, Axe2x80x3, Axe2x80x2xe2x80x3, Axe2x80x3xe2x80x3 and Axe2x80x2xe2x80x3xe2x80x3 are preferably Gd, La and Sr, and B, Bxe2x80x2, Bxe2x80x3, Bxe2x80x2xe2x80x3, Bxe2x80x3xe2x80x3 and Bxe2x80x2xe2x80x3xe2x80x3 are preferably Mn, Cu, Co, Cr and Fe. Preferably, p and q are less than 50%. Preferably m is 10% to 50%. Preferably n is 5% or less.
Specific examples of compounds of this class include those with formula CeO2xe2x88x92xcex4/10% CuO and CeO2xe2x88x92xcex4/10% MnO2.
More preferred catalysts of the invention are mixtures of cerium and/or zirconium with one or more metals selected from the group consisting of: Pt, Pd, Cu, Gd, Mn, Fe, Zr, La, Co, Sr, Au, and Cr. Preferably, these metals are present in a concentration of about 1% to 50%.
Also provided are methods for reducing the concentration of VOCs in an oxygen-containing gas containing at least one VOC which comprises the step of contacting said gas with a catalyst composition of the invention, whereby the concentration of at least one VOC in said gas is reduced. These methods may further comprise heating either said gas or said catalyst composition, or both, to a temperature sufficient to reduce the concentration of at least one VOC in said gas to a selected value. The catalyst compositions may be held in a reactor at temperatures of from about ambient temperature to about 250xc2x0 C. Preferably, the temperatures are from about ambient temperature to about 150xc2x0 C. The gases may contain the normal components of air, hydrocarbons and halogen-containing materials. The gas may also contain substances including water vapor, sulfur-containing gases and other substances.
Catalyst compositions useful in the methods of the invention include those with a surface area ranging from about 20 to about 220 m2/g.
The catalyst compositions useful in the methods of the invention may be prepared by methods known in the art, or the methods described herein or modifications of methods known in the art or modifications of the methods described herein, including the method comprising: (a) treating a mixture of selected metal salt precursors with a precipitating reagent to form a precipitate; (b) drying said precipitate; (c) calcining said precipitate at a temperature of about 300xc2x0 C. or higher; (d) contacting one or more solutions of one or more metals selected from the group consisting of Pt, Pd, Rh, Ru, Re, Os and Ir (preferably Pt or Pd or mixtures thereof) with said precipitate; (e) reducing said composition (preferably at temperatures of about 200xc2x0 C. or higher); and optionally (f) oxidizing said composition (preferably at temperatures of between about 80xc2x0 C. to about 800xc2x0 C.). The catalyst compositions useful in the methods of the invention may also be prepared by the method comprising the steps: (a) mixing selected metal oxides; (b) ball milling said metal oxides; and (c) calcining in air (preferably at temperatures of about 300xc2x0 C. or higher).
One class of catalyst compositions of the invention include combinations of ceria and/or zirconia optionally combined with platinum and/or palladium. Another class of catalyst compositions of the invention include combinations of ceria and/or zirconia at least one metal from the group consisting of: Gd, La, Sc, Cr, Ti, V, Mn, Fe, Co, Ni, Au, Ag and Cu, optionally combined with platinum and/or palladium. Catalyst compositions of the invention include those with predominantly fluorite crystal structures. Other structures that may be present include defect fluorite, pyrochlore (A2B2O7) and perovskite-like phases. Cerium and zirconium oxide with some amount of dopants are generally present as fluorite structures. Dopants may also be present as oxides. A class of catalyst compositions of the invention are those that do not require an inert support, such as alumina or carbon. Another class of catalyst compositions of the invention include a support material such as a honeycomb matrix having inner and outer surfaces, wherein said catalyst material is present on the inner surfaces of said honeycomb matrix. Preferably, the support material is fabricated from ceramic materials, but may also be fabricated from metals or ceramic or metal fibers.
The catalyst compositions may be coated onto the support material by any method which produces a suitable coating of catalyst composition, including the method of: (a) treating a mixture of metal salt precursors with a precipitating reagent to form a precipitate; (b) preparing a slurry of said precipitate; (c) coating said slurry onto said support; and (d) calcining said slurry. The catalyst compositions may also be coated onto a support material by: (a) mixing a solution of metal salt precursors with the support; and (b) calcining said precursors. The catalyst compositions may also be coated onto said support material by: (a) mixing the support with one or more metal salt precursors to form a mixture; (b) treating said mixture with a precipitating reagent to form a precipitate; and (c) calcining said precipitate.
The methods above may also further comprise the steps of: adding one or more metals selected from the group consisting of Pt, Pd, Rh, Ru, Re, Os and Ir metal to form a composition; reducing said composition (preferably at temperatures of about 200xc2x0 C. or higher); and optionally oxidizing said composition (preferably at temperatures of between about 80xc2x0 C. to about 800xc2x0 C.).
Also provided are methods of decomposition of VOCs in a gas, comprising the steps of providing a reactor containing a catalyst composition of the invention; and passing the gas through the reactor to decompose the VOCs. Also provided is a catalytic reactor for decomposition of VOCs in a gas which comprises: a casing having an entrance port, an exit port and a passage therebetween for the movement of said gases from said entrance port to said exit port with a catalyst composition of the invention in said passage. In the catalytic reactor, the gases preferably contact said catalyst before exiting said casing.
In the catalyst compositions of the invention, if a metal selected from the group consisting of: Pt, Pd, Rh, Ru, Re, Os and Ir and mixtures thereof is present, the metal(s) is preferably dispersed onto the metal oxide surface, and preferably the incipient wetness impregnation method is used.
As used herein, xe2x80x9ccatalyst compositionxe2x80x9d includes those compositions useful for destruction of VOCs in a gas. Catalyst compositions comprise at least one of cerium and zirconium with at least one member of the group consisting of: Gd, La, Sc, Cr, Ti, V, Mn, Fe, Co, Ni, Au, Ag, Cu Pt, Pd, Rh, Ru, Re, Os and Ir. Catalyst compositions of the invention are useful in reducing the concentration of at least one VOC in gas. As used herein, xe2x80x9cdestructionxe2x80x9d of a VOC refers to the transformation of a VOC into another substance, preferably carbon dioxide and water. As used herein, xe2x80x9cmixed metal oxidesxe2x80x9d include one or more metal oxides. As used herein, xe2x80x9csingle-phase materialxe2x80x9d is a material that comprises a single crystallographic phase. As used herein, xe2x80x9cmulti-phase materialxe2x80x9d refers to a material wherein some components are single-phase and other components are mixed metal oxides. As used herein, a xe2x80x9cprecipitating reagentxe2x80x9d is a substance or mixture of substances that causes precipitation of a desired substance. Preferred precipitating reagents include NH4OH, (NH4)2CO3, Na2CO3, NaOH, urea and K2CO3. As used herein, xe2x80x9ccontactingxe2x80x9d substances is meant to indicate that substances are physically near each other, but is not intended to mean a homogeneous solution is formed. Preferably, reducing occurs in a hydrogen-containing atmosphere and oxidation occurs in air, but any reaction conditions that produce the desired result may be used.
The catalysts of this invention are suitable for use in any reactor system and particularly with either fixed and fluid bed reactors and can be prepared as powders or pressed into plugs, pellets and other shapes suitable for use in a given reactor configuration.
VOCs destroyed by the catalyst compositions of the invention include, but are not limited to: acetates, alkanes, alkenes, alcohols, aldehydes, ethers, esters, aromatics, carboxylic acids, ketones, and halogenated hydrocarbons.
The catalyst compositions of the invention may be used to reduce the concentration of VOCs in a gaseous atmosphere. Preferably, the VOCs are converted to carbon dioxide and water. Preferably, the gaseous atmosphere is oxygen-containing, and most preferably, the gaseous atmosphere is air. Catalyst composition of the invention may be used to reduce the concentration of VOCs in gases containing any concentration of VOCs. In one application, catalyst compositions of the invention may reduce the concentration of VOCs in air where there is an excess of oxygen and between between 10 ppm and percent levels of VOCs.
The catalysts of the invention are all useful to reduce the concentration of VOCs in gases at low temperatures (from about 200xc2x0 C. and below). Preferably, temperatures from about 150xc2x0 C. to about 30xc2x0 C. are used. Most preferably, temperatures of about 100xc2x0 C. and below are used.
The catalysts of the invention function in the presence of potentially interfering substances, such as water, sulfur-containing gases and halogens.
The catalysts are preferably preconditioned prior to said gases contacting said catalyst. The preconditioning treatment is useful to desorb moisture and change the oxidation state of some species. More preferably, the catalysts are preconditioned at a temperature of between about 150xc2x0 C. to 400xc2x0 C. Catalyst compositions of the invention are preferably preconditioned under a flow of air or VOC for a time sufficient to maximize activity, preferably for one hour or more. The preferred preconditioning time is longer at lower temperatures and can be as long as 24 hours at temperatures of 100xc2x0 C. or less.
Catalysts of the invention have long lifetimes and can be regenerated by heating for a sufficient time to drive off adsorbed organics and moisture. For example, catalysts of the invention may be regenerated by heating at a temperature of about 150xc2x0 C.